Zirconia sintered body and method for manufacturing the same

ABSTRACT

A method for manufacturing a zirconia sintered body includes molding a powder composition that has a yttria content of more than 3% by mole and 5.2% by mole or less and that contains a first zirconia powder having a yttria content of 2% by mole or more and 4% by mole or less and a second zirconia powder having a yttria content of more than 4% by mole and 6% by mole or less to obtain a green body, and sintering the green body to obtain a sintered body.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional application of U.S. patent application Ser. No.16/313,393, filed Dec. 26, 2018 which is a U.S. National Phase under 35U.S.C. § 371 of International Patent Application No. PCT/JP2018/010748,filed Mar. 19, 2018. The disclosure of each of the above-mentioneddocuments, including the specification, drawings, and claims, isincorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention relates to a zirconia sintered body having bothtranslucency and high strength and a method for manufacturing the same.

BACKGROUND ART

Zirconia sintered bodies are used not only as structural materials anddecorative members but also as dental materials, since they have highmechanical strength and high aesthetic properties due to theirtranslucency (PTL 1 to PTL 4). A zirconia sintered body used as a dentalmaterial is required to have a high translucency of a total lighttransmittance of 40% or more and a high bending strength of 800 MPa ormore (PTL 1).

An example of the zirconia sintered body that satisfies such arequirement, a zirconia sintered body manufactured by hot isostaticpressing (hereinafter, referred to as “HIP”) and having a total lighttransmittance of 43% or more at a sample thickness of 0.5 mm, athree-point bending strength of 1,700 MPa or more and an yttria contentof 2% to 4% by mole has been disclosed (PTL 2). However, the practicaluse of the zirconia sintered body disclosed in PTL 2 is limited becauseHIP requires a large apparatus.

On the other hand, studies have been conducted on a zirconia sinteredbody having both high mechanical strength and high translucency andcapable of being obtained without HIP and a method for manufacturing thezirconia sintered body.

PTL 1 discloses a method for molding and sintering a zirconia powderwhich has an yttria content of about 3% by mole and in which the BETspecific surface area and crystallinity are controlled. PTL 1 disclosesthat a zirconia sintered body having a total light transmittance of41.20% to 44.06% to D65 light at a sample thickness of 0.5 mm and abending strength of 860 to 891 MPa is obtained by firing at a maximumtemperature of 1,450° C. for 2 hours. However, the total lighttransmittance of this zirconia sintered body is only less than 30% atmaximum when converted into a total light transmittance to D65 light ata sample thickness of 1 mm (hereinafter, also simply referred to as a“total light transmittance”). The zirconia sintered body disclosed inPTL 1 does not have translucency sufficient for use as a dentalmaterial.

Regarding a zirconia sintered body having mechanical strength andtranslucency that are high enough to be practically used as a dentalmaterial, PTL 3 discloses that a zirconia sintered body is obtained bypressureless sintering of a zirconia powder which has an yttria contentof 2% to 4% by mole and in which a sintering shrinkage rate from arelative density of 70% to a relative density of 90%, a BET specificsurface area, etc. are controlled, and the zirconia sintered body has atotal light transmittance of 35% or more and a three-point bendingstrength of 1,000 MPa or more.

PTL 4 discloses a zirconia sintered body having both high mechanicalstrength and high translucency and having translucency high enough to beapplied to a dental material for a front tooth, and a method formanufacturing such a zirconia sintered body without HIP. The zirconiasintered body disclosed in PTL 4 is a zirconia sintered body obtained bypressureless sintering and having an yttria content of more than 4% bymole and 6.5% by mole or less and has a total light transmittance of 42%or more and a three-point bending strength of 550 to 870 MPa.

The zirconia sintered bodies disclosed in PTL 3 and PTL 4, which can bemanufactured by pressureless sintering, are used as practical dentalmaterials because they have high mechanical strength and hightranslucency.

CITATION LIST Patent Literature

PTL 1: Japanese Unexamined Patent Application Publication No.2016-108176

PTL 2: Japanese Unexamined Patent Application Publication No.2008-050247

PTL 3: Japanese Unexamined Patent Application Publication No.2010-150063

PTL 4: Japanese Unexamined Patent Application Publication No.2015-143178

SUMMARY OF INVENTION Technical Problem

The zirconia sintered bodies disclosed in PTL 3 and PTL 4 satisfymechanical strength and translucency that are required for dentalmaterials. However, in an actual use, different dental materials areappropriately used depending on the application position, for example,among dental materials, a material having higher strength is used as adental material for a back tooth, and a dental material having highertranslucency is used as a dental material for a front tooth. PTL 4discloses only a single zirconia sintered body having both highmechanical strength and high translucency, specifically, having athree-point bending strength of 870 MPa and a total light transmittanceof 44%. However, in order to repeatedly manufacture this zirconiasintered body, it is necessary to control manufacturing conditions inpainstaking detail.

In view of these problems, an object of the present invention is toprovide a zirconia sintered body capable of being widely used as adental material, in particular, a zirconia sintered body capable ofbeing applied to both a dental material for a back tooth and a dentalmaterial for a front tooth and a simple and easy method formanufacturing the zirconia sintered body.

Solution to Problem

The inventors of the present invention conducted studies on a zirconiasintered body having both mechanical properties and translucency thatsatisfy properties required for a dental material, in particular, both adental material for a front tooth and a dental material for a back toothand a method for manufacturing the zirconia sintered body. As a result,it was found that such a zirconia sintered body can be simply and easilymanufactured by controlling a state of a raw material powder, and thisfinding led to the completion of the present invention.

Specifically, the gist of the present invention is as follows.

-   [1] A zirconia sintered body having an yttria content of more than    3% by mole and 5.2% by mole or less and an yttrium concentration    distribution in which a maximum of a frequency is 7.5% or less, the    yttrium concentration distribution being obtained by a quantitative    analysis of elements of an energy dispersive X-ray spectrum.-   [2] The zirconia sintered body according to [1], in which a crystal    phase of the zirconia sintered body comprises a tetragonal phase    including a T phase and a T* phase.-   [3] The zirconia sintered body according to [1] or [2], in which the    zirconia sintered body has an average crystal grain size of more    than 0.41 μm and 1.5 μm or less.-   [4] A method for manufacturing a zirconia sintered body, the method    comprising a molding step of molding a powder composition that has    an yttria content of more than 3% by mole and 5.2% by mole or less    and that contains a first zirconia powder having an yttria content    of 2% by mole or more and 4% by mole or less and a second zirconia    powder having an yttria content of more than 4% by mole and 6% by    mole or less to obtain a green body; and a sintering step of    sintering the green body to obtain a sintered body.-   [5] The manufacturing method according to [4], in which the powder    composition contains alumina.-   [6] The manufacturing method according to [4] or [5], in which the    powder composition has a BET specific surface area of 5 m²/g or more    and less than 17 m²/g.-   [7] The manufacturing method according to any one of [4] to [6], in    which a weight ratio of the first zirconia powder to the second    zirconia powder is 35% by weight:65% by weight to 65% by weight:35%    by weight.-   [8] The manufacturing method according to any one of [4] to [7], in    which the first zirconia powder has an yttria content of 2% by mole    or more and 3.5% by mole or less, and the second zirconia powder has    an yttria content of 4.5% by mole or more and 5.7% by mole or less.-   [9] The manufacturing method according to any one of [4] to [8], in    which a sintering temperature in the sintering step is 1,400° C. or    higher and 1,600° C. or lower.

Advantageous Effects of Invention

The present invention can provide a zirconia sintered body capable ofbeing widely used as a dental material, in particular, a zirconiasintered body capable of being applied to both a dental material for aback tooth and a dental material for a front tooth and a manufacturingmethod by which such a zirconia sintered body can be simply and easilymanufactured.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A graph showing the yttrium concentration distribution of Example3.

FIG. 2A graph showing the yttrium concentration distribution ofComparative Example 2.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of a method for manufacturing a zirconiasintered body according to the present invention will be described.

A manufacturing method according to an embodiment of the presentinvention comprises a molding step of molding a powder composition thathas an yttria content of more than 3% by mole and 5.2% by mole or lessand that contains a first zirconia powder having an yttria content of 2%by mole or more and 4% by mole or less and a second zirconia powderhaving an yttria content of more than 4% by mole and 6% by mole or less(hereinafter, also simply referred to as a “powder composition”) toobtain a green body.

The powder composition is used in the molding step. The powdercomposition has an yttria content of more than 3% by mole and 5.2% bymole or less. When the yttria content is 3% by mole or less, theresulting zirconia sintered body has a total light transmittance of 42%or less to D65 light at a sample thickness of 1 mm, and thus the rangeof applications to dental materials is limited. In contrast, when theyttria content exceeds 5.2% by mole, a zirconia sintered body having athree-point bending strength of 800 MPa or more, which is required for adental material, is not obtained even by the manufacturing methodaccording to an embodiment of the present invention.

The yttria content of the powder composition is 3.5% by mole or more and4.8% by mole or less, preferably 3.8% by mole or more and 4.6% by moleor less, and preferably 3.8% by mole or more and 4.3% by mole or less.

The powder composition contains a first zirconia powder having an yttriacontent of 2% by mole or more and 4% by mole or less (hereinafter, alsoreferred to as a “low yttria powder”) and a second zirconia powderhaving an yttria content of more than 4% by mole and 6% by mole or less(hereinafter, also referred to as a “high yttria powder”). The zirconiasintered body obtained by sintering such a powder composition has atendency that the mechanical strength improves with an increase in theaverage crystal grain size. Therefore, a zirconia sintered body havingboth mechanical strength and translucency that are equal to or higherthan the properties required for a dental material can be obtainedwithout controlling manufacturing conditions in detail.

The yttria content of the low yttria powder is 2% by mole or more and 4%by mole or less, preferably 2% by mole or more and 3.5% by mole or less,and more preferably 2.3% by mole or more and 3.5% by mole or less. Theyttria content of the high yttria powder is more than 4% by mole and 6%by mole or less, preferably 4% by mole or more and 5.7% by mole or less,and more preferably 4.5% by mole or more and 5.7% by mole or less.

In an embodiment of the present invention, the powder compositionpreferably contains a zirconia powder having an yttria content of 2.3%by mole or more and 3.5% by mole or less as the low yttria powder and azirconia powder having an yttria content of 5.2% by mole or more and5.7% by mole or less as the high yttria powder.

The content ratio of the low yttria powder to the high yttria powder ofthe powder composition is not limited as long as the powder compositionhas a desired yttria content. The ratio may be low yttria powder:highyttria powder=1% by weight:99% by weight to 99% by weight:1% by weightin terms of weight ratio of the low yttria powder to the high yttriapowder, though the content ratio of the low yttria powder to the highyttria powder of the powder composition varies depending on the yttriacontents of the powders. The weight ratio is more preferably low yttriapowder:high yttria powder=20% by weight:80% by weight to 80% byweight:20% by weight, and still more preferably 35% by weight:65% byweight to 65% by weight:35% by weight.

Each of the low yttria powder and the high yttria powder can bemanufactured by a manufacturing method including a hydrolysis step ofobtaining a hydrated zirconia sol by hydrolysis of an aqueous solutionof a zirconium salt, a drying step of mixing an yttrium compound to theresulting hydrated zirconia sol and then drying the resulting mixture toobtain a dry powder, and a calcination step of calcining the dry powderto obtain a calcined powder. A preferred method for manufacturing a lowyttria powder and a high yttria powder, for example, is a manufacturingmethod including mixing at least one of yttrium oxide or yttriumchloride with a hydrated zirconia sol obtained by hydrolyzing at leastone selected from the group consisting of zirconium oxychloride,zirconyl nitrate, zirconium chloride and zirconium sulfate, subsequentlydrying the resulting mixture in air at 80° C. or higher and 200° C. orlower, and subsequently calcining the dry mixture in air at 1,050° C. orhigher and 1,250° C. or lower.

The powder composition may contain alumina (Al₂O₃). The alumina contentof the powder composition is 0% by weight or more and 0.1% by weight orless, preferably 0% by weight or more and less than 0.1% by weight, andmore preferably 0% by weight or more and 0.075% by weight or less interms of the ratio of the weight of alumina to the weight of the powdercomposition. In the case where the powder composition contains alumina,the alumina content is more than 0% by weight and 0.1% by weight orless, preferably more than 0% by weight and less than 0.1% by weight,more preferably more than 0% by weight and 0.075% by weight or less, andstill more preferably 0.01% by weight or more and 0.075% by weight orless in terms of the ratio of the weight of alumina to the weight of thepowder composition.

The crystallite size of the powder composition is 200 Å or more and 400Å or less, and preferably 300 Å or more and 400 Å or less. Thecrystallite size of the powder composition can be obtained from an XRDpeak (hereinafter, also referred to as a “main XRD peak”) of thetetragonal (111) plane and the cubic (111) plane in powder X-raydiffraction (hereinafter, referred to as “XRD”) measurement by using theformula below.

Crystallite size=κλ/β cos θ

In the above formula, κ represents a Scherrer constant (κ=1), λrepresents the wavelength of the measured X-ray (λ=1.541862 Å when CuKαrays are used as a radiation source), β represents a full width)(°) athalf maximum of the main XRD peak, and θ represents the Bragg angle ofthe main XRD peak.

Note, that the main XRD peak is an XRD peak having a peak top at2θ=30.1° to 30.2° in XRD in which CuKα rays are used as a radiationsource. The peak is an XRD peak in which the tetragonal (111) plane andthe cubic (111) plane overlap each other. To calculate the crystallitesize, the main XRD peak is subjected to waveform processing withoutseparating the tetragonal and cubic phase peaks. The Bragg angle (θ) ofthe main XRD peak after the waveform processing and the full width (β)at half maximum of the main XRD peak after mechanical spreading widthcorrection are determined.

In the powder composition, a ratio of the tetragonal phase and the cubicphase (hereinafter, also referred to as “T+C phase ratio”) obtained bythe formula below is preferably 60% or more, and more preferably 65% ormore.

T+C phase ratio (%)=100−fm (%)

In the above formula, fm represents a monoclinic phase ratio obtainedfrom an XRD pattern.

Preferably, the powder composition, the low yttria powder and the highyttria powder have substantially the comparable BET specific surfacearea and average particle size from the viewpoint that the physicalproperties of the powder composition are made homogeneous.

A preferred BET specific surface area may be 5 m²/g or more and lessthan 17 m²/g. The BET specific surface area is preferably 5 m²/g or moreand 15 m²/g or less. A preferred average particle size may be 0.30 μm ormore and 0.60 μm or less. The average particle size is preferably 0.35μm or more and 0.55 μm or less, and more preferably 0.40 μm or more and0.50 μm or less.

The powder composition may be manufactured by any method as long as thepowder composition contains a low yttria powder and a high yttria powderand the powder composition having a desired composition is obtained. Thepowder composition is simply and easily obtained by mixing a low yttriapowder and a high yttria powder. Any mixing method may be employed aslong as the two yttria powders are uniformly mixed. At least one of drymixing or wet mixing, and furthermore, wet mixing can be employed. Apreferred mixing method is mixing in a water solvent. For the purpose ofuniform mixing, non-granulated powders, such as calcined powders, arepreferably used as the low yttria powder and the high yttria powder.

In the case the powder composition contains alumina, the powdercomposition may be obtained by mixing the low yttria powder and the highyttria powder, and subsequently mixing the resulting mixed powder withalumina. However, the powder composition is preferably obtained bymixing alumina with at least one of the low yttria powder or the highyttria powder, and subsequently mixing the low yttria powder and thehigh yttria powder.

The powder composition used in the molding step is preferably agranulated powder. A granulated powder composition has high fluidity andtherefore is easily densified during sintering. Examples of propertiesof a granulated powder include an average granule size of 30 μm or moreand 80 μm or less and a bulk density of 1.10 g/cm³ or more and 1.40g/cm³ or less.

The granulated powder is obtained by mixing a powder composition and anorganic binder, and subjecting the resulting mixture to spray-drying.The content of the organic binder of the powder composition may be 1% byweight or more and 5% by weight or less relative to the weight of thepowder composition.

In the molding step, any molding method may be employed as long as agreen body having a desired shape is obtained. The molding method is,for example, at least one selected from the group consisting of pressmolding, cold isostatic pressing, sheet molding and injection molding.

The green body may have any shape. The shape may be at least oneselected from the group consisting of a spherical shape, a substantiallyspherical shape, an elliptical shape, a disk shape, a columnar shape, acubic shape, a rectangular parallelepiped shape, a polyhedral shape anda substantially polyhedral shape.

The manufacturing method according to an embodiment of the presentinvention may include a pre-sintering step of pre-sintering the greenbody obtained in the molding step to obtain a pre-sintered body. Thepre-sintering step is a step between the molding step and a sinteringstep. The pre-sintering step provides a zirconia pre-sintered bodyhaving an yttria content of more than 3% by mole and 5.2% by mole orless, preferably 3.5% by mole or more and 4.8% by mole or less, morepreferably 3.8% by mole or more and 4.6% by mole or less, and still morepreferably 3.8% by mole or more and 4.3% by mole or less. Thepre-sintered body is a green body at an initial stage of sintering andhas a structure including necking between particles. In thepre-sintering step, the pre-sintered body may be processed so as to haveany shape.

The pre-sintering can be conducted by heat treatment at a pre-sinteringtemperature of 600° C. or higher and 1,400° C. or lower, preferably 600°C. or higher and lower than 1,400° C., more preferably 600° C. or higherand 1,200° C. or lower, and still more preferably 800° C. or higher and1,100° C. or lower.

The holding time at the pre-sintering temperature is, for example, 1hour or more and 5 hours or less, preferably 1 hour or more and 3 hoursor less, and more preferably 1 hour or more and 2 hours or less.

The pre-sintering atmosphere should be an atmosphere other than areducing atmosphere and is preferably at least any of an oxygenatmosphere or an air atmosphere. An air atmosphere is simply and easilyused.

In the sintering step, the green body obtained in the molding step issintered to obtain a sintered body. This step provides a zirconiasintered body having an yttria content of more than 3% by mole and 5.2%by mole or less, preferably 3.5% by mole or more and 4.8% by mole orless, more preferably 3.8% by mole or more and 4.6% by mole or less, andstill more preferably 3.8% by mole or more and 4.3% by mole or less.

When the manufacturing method of the present invention includes thepre-sintering step, the pre-sintered body may be sintered instead of thegreen body in the sintering step.

The sintering temperature in the sintering step is 1,400° C. or higherand 1,600° C. or lower, preferably 1,420° C. or higher and 1,580° C. orlower, more preferably 1,440° C. or higher and 1,560° C. or lower, andparticularly preferably 1,480° C. or higher and 1,560° C. or lower. Thesintering temperature in another embodiment is 1,450° C. or higher and1,650° C. or lower, preferably 1,500° C. or higher and 1,650° C. orlower, more preferably 1,500° C. or higher and 1,650° C. or lower, andstill more preferably 1,550° C. or higher and 1,650° C. or lower.

The temperature-increasing rate in the sintering step may be 150° C./hor more and 800° C./h or less and is preferably 150° C./h or more and700° C./h or less, and more preferably 200° C./h or more and 600° C./hor less. Under such a condition, the green body can be sintered at thesintering temperature while suppressing progress of sintering in thetemperature-increasing process.

The holding time at the sintering temperature (hereinafter, also simplyreferred to as a “holding time”) varies depending on the sinteringtemperature. The holding time is, for example, 1 hour or more and 5hours or less, preferably 1 hour or more and 3 hours or less, and morepreferably 1 hour or more and 2 hours or less.

The sintering atmosphere should be an atmosphere other than a reducingatmosphere and is preferably at least any of an oxygen atmosphere or anair atmosphere. An air atmosphere is simply and easily used.

A particularly preferred sintering step may include sintering atatmospheric pressure, at a temperature-increasing rate of 700° C./h orless and at a sintering temperature of 1,440° C. or higher and 1,560° C.or lower.

In the sintering step, any sintering method may be employed. Thesintering method may be at least one selected from the group consistingof pressureless sintering, HIP, PSP, and vacuum sintering. A typicalmeans for enhancing translucency is to use, after a sintered body isobtained by pressureless sintering, a special sintering method such asHIP or another pressure sintering method or SPS. However, such specialsintering methods not only complicate the manufacturing process, butalso cause an increase in the manufacturing cost. Accordingly, thesintering step in an embodiment of the present invention preferablyincludes pressureless sintering, and more preferably includes onlypressureless sintering. The term “pressureless sintering” refers to amethod of sintering a green body by simply heating the green bodywithout applying any external force during sintering. An specificexample of pressureless sintering is sintering under atmosphericpressure.

One feature in the manufacturing method according to an embodiment ofthe present invention is that, with an increase in the sinteringtemperature, a decrease in mechanical strength due to an increase in theaverage crystal grain size of the resulting zirconia sintered body tendsto be significantly suppressed, and furthermore, with an increase in theaverage crystal grain size, mechanical strength increases in some cases.Accordingly, a zirconia sintered body having a higher total lighttransmittance while maintaining high mechanical strength is easilyobtained. Therefore, a zirconia sintered body having both mechanicalproperties and translucency that satisfy properties required for both adental material for a front tooth and a dental material for a back toothcan be simply and easily manufactured without controlling manufacturingconditions in detail.

According to another aspect of the present invention, an embodiment ofthe present invention may be a method for manufacturing a zirconiapre-sintered body, the method including a molding step of molding apowder composition that contains a first zirconia powder having anyttria content of 2% by mole or more and 4% by mole or less and a secondzirconia powder having an yttria content of more than 4% by mole and 6%by mole or less and that has an yttria content of more than 3% by moleand 5.2% by mole or less to obtain a green body; and pre-sintering thegreen body to obtain a pre-sintered body.

The zirconia sintered body according to an embodiment of the presentinvention is a sintered body having a wider yttria concentrationdistribution than existing sintered bodies. Specifically, the sinteredbody according to an embodiment of the present invention is a zirconiasintered body having an yttria content of more than 3% by mole and 5.2%by mole or less and an yttrium concentration distribution in which amaximum of a frequency is 7.5% or less, the yttrium concentrationdistribution being obtained by a quantitative analysis of elements of anenergy dispersive X-ray spectrum.

The sintered body according to an embodiment of the present inventionhas an yttria content of 3.5% by mole or more and 4.8% by mole or less,preferably 3.8% by mole or more and 4.6% by mole or less, and morepreferably 3.8% by mole or more and 4.3% by mole or less.

In an embodiment of the present invention, the yttria content of thezirconia sintered body is an average composition of the sintered bodyobtained by the composition analysis and can be measured by ICPanalysis.

In the sintered body according to an embodiment of the presentinvention, the maximum of a frequency of an yttrium concentrationdistribution in a quantitative analysis of elements of an energydispersive X-ray spectrum (hereinafter, also simply referred to as a“frequency”) is 7.5% or less and preferably 7.0% or less.

A maximum of the frequency (hereinafter, also referred to as a “maximumfrequency”) of 7.5% or less means that the sintered body according to anembodiment of the present invention has a large number of regions havingdifferent yttria concentrations therein. The yttria content in the aboverange and this yttrium concentration distribution of the sintered bodyare believed to be one reason for contribution to an improvement inmechanical properties of the sintered body. The maximum frequency may be3.0% or more, and further 5.0% or more.

The quantitative analysis of elements of an EDS spectrum is performed byobtaining an EDS spectrum of a scanning electron microscope(hereinafter, also referred to as a “SEM”) observation image of asintered body, and quantifying the intensities of characteristic X-raysof zirconium (Zr) and yttrium (Y) of the EDS spectrum. Thequantification of the EDS spectrum may include measurement at 40,000positions or more that are selected at random on the SEM observationimage. The yttrium concentration is preferably obtained as a ratio ofthe intensity of yttrium to the intensity of zirconium (hereinafter,also referred to as a “Y/Zr ratio”). The yttrium concentrationdistribution is preferably an yttrium concentration distribution dividedinto regions each corresponding to a certain yttrium concentration rangeand may be an yttrium concentration distribution divided intoconcentration ranges at intervals of 0.5% in terms of the Y/Zr ratio.The frequency is a ratio (%) of the number of measurement pointscorresponding to each yttrium concentration relative to the total numberof measurement points in the quantification of the EDS spectrum. Thesintered body according to an embodiment of the present invention has ahomogeneous composition on the surface and inside thereof. Therefore,the yttrium concentration distribution of the whole sintered body can bemeasured by EDS observation of the surface.

The crystal phase of the sintered body according to an embodiment of thepresent invention comprises a tetragonal phase. The tetragonal phasepreferably includes the T phase and the T* phase. A ratio of the T*phase to the T phase in the crystal phase (hereinafter, also referred toas “T*/T ratio”) is 62% or more and less than 100%, and preferably 65%or more and less than 100%. More preferably, the T*/T ratio is 76% ormore and less than 100%, and further 76% or more and 90% or less becausethe mechanical strength increases.

The sintered body of an embodiment of the present invention may containalumina (Al₂O₃). The alumina content of the sintered body of anembodiment of the present invention may be 0% by weight or more and 0.1%by weight or less, and is preferably 0% by weight or more and less than0.1% by weight, and more preferably 0% by weight or more and 0.075% byweight or less, in terms of a ratio of the weight of alumina to theweight of the sintered body according to an embodiment of the presentinvention. When the sintered body according to an embodiment of thepresent invention contains alumina, the alumina content may be more than0% by weight and 0.1% by weight or less, and is preferably more than 0%by weight and less than 0.1% by weight, more preferably more than 0% byweight and 0.075% by weight or less, and still more preferably 0.01% byweight or more and 0.075% by weight or less, in terms of a ratio of theweight of alumina to the weight of the sintered body according to anembodiment of the present invention.

Mechanical strength and translucency of a zirconia sintered body areaffected by the average crystal grain size, and these properties have atrade-off relationship. Specifically, when the average crystal grainsize is decreased, mechanical strength increases but translucencydecreases. On the other hand, when the average crystal grain size isincreased, mechanical strength decreases but translucency increases. Inorder to realize both mechanical strength and translucency that arerequired for a dental material, hitherto, high translucency is achievedby controlling the yttria content to more than 3% by mole, and highmechanical strength is achieved by controlling the average crystal grainsize to about 0.41 μm.

In contrast, even when the sintered body according to an embodiment ofthe present invention has an average crystal grain size of more than0.41 μm, further 0.42 μm or more, and further 0.45 μm or more, thezirconia sintered body has mechanical strength equal to or higher thanthe mechanical strength required fora dental material. The averagecrystal grain size of the sintered body according to an embodiment ofthe present invention may be 1.5 μm or less and is preferably 1.0 μm orless. The average crystal grain size is preferably 0.55 μm or more and1.5 μm or less and more preferably 0.6 μm or more and 1.0 μm or less.

The sintered body according to an embodiment of the present invention ispreferably a zirconia sintered body having an yttria content of morethan 3% by mole and 5.2% by mole or less and an average crystal grainsize of 0.42 μm or more. The zirconia sintered body is a so-calledtranslucent zirconia sintered body having higher mechanical strength andhigher translucency than the mechanical strength and translucency thatare required for a dental material.

The sintered body according to an embodiment of the present invention isobtained by pressureless sintering and has high mechanical strengthwhile having the above large average crystal grain size. Regarding themechanical strength of the sintered body according to an embodiment ofthe present invention, the three-point bending strength obtained inaccordance with the measuring method described in JIS R 1601 may be 810MPa or more and is preferably more than 870 MPa, and more preferably 900MPa or more. In particular, the sintered body according to an embodimentof the present invention has a high three-point bending strength of 900MPa or more and 1,300 MPa or less, and further 900 MPa or more and 1,200MPa or less even when the yttria content is 3.8% by mole or more and4.2% by mole or less, and further 3.8% by mole or more and 4.15% by moleor less.

The sintered body according to an embodiment of the present inventionhas the above mechanical strength and a total light transmittance of 43%or more, preferably more than 44%, and more preferably 44.5% or more. Atan yttria content of more than 3% by mole and 5.2% by mole or less, thetotal light transmittance may be 49% or less. The total lighttransmittance in embodiments of the present invention can be measured inaccordance with the method described in JIS K 7361 with a D65 lightsource by using a sintered body having a sample thickness of 1 mm.

Such a zirconia sintered body can be provided as a zirconia sinteredbody having mechanical strength required for a dental material withoutimpairing translucency of existing zirconia sintered bodies for dentalmaterials and further having higher translucency than translucency ofexisting zirconia sintered bodies for dental materials. Thus, thezirconia sintered body according to an embodiment of the presentinvention can be used as a dental material and can be further used asboth a dental material for a back tooth and a dental material for afront tooth.

EXAMPLES

The present invention will be specifically described by way of Exampleshereinafter. However, the present invention is not limited to theseExamples.

Methods for measuring characteristics of sintered bodies and powdersaccording to the present invention will be described below.

(Total Light Transmittance)

The total light transmittance was measured in accordance with the methoddescribed in JIS K 7361 by using a turbidimeter (device name: NDH2000,available from Nippon Denshoku Industries Co., Ltd.) with a D65 lightsource.

A disk-shaped sintered body having a thickness of 1 mm and polished twosurfaces was used as a measurement sample.

(Three-Point Bending Strength)

The bending strength was measured by a three-point bending test based onJIS R 1601 “Testing method for flexural strength of fine ceramics”. Themeasurement was conducted 10 times, and the average was defined as athree-point bending strength. The measurement was conducted at adistance between supported points of 30 mm by using a pillar-shapedsintered body sample having a width of 4 mm and a thickness of 3 mm.

(Sintered Body Density)

A measured density of a sintered body was obtained in accordance withthe measuring method described in JIS R 1634 (Test methods for densityand apparent porosity of fine ceramics). A relative density was obtainedfrom a ratio of the measured density to a theoretical density. Prior tothe measurement of the measured density, a pretreatment was performed bymeasuring the mass of the sintered body after drying, subsequentlyputting the sintered body into water, and boiling the sintered body forone hour.

The theoretical density (ρ₀) was obtained by formula below.

ρ₀=100/[(A/ρ_(A))+(100−A)/ρ_(X)]

In the above formula, ρ₀ represents a theoretical density (g/cm³), Arepresents a content (% by weight) of Al₂O₃, ρ_(A) represents atheoretical density (3.99 g/cm³) of Al₂O₃, and ρ_(X) represents atheoretical density (g/cm³) of an X mol % yttria-containing zirconiasintered body.

ρ_(X) in the above formula represents different values depending on thecrystal phase of the zirconia sintered body. Herein, a value calculatedfrom the formula described in J. Am. Ceram. Soc., 69 [4] 325-32 (1986)(hereinafter, also referred to as a “reference document”) can be used asthe theoretical density ρ_(X).

(Average Crystal Grain Size)

The average crystal grain size of a sintered body sample was obtained bythe planimetric method from a SEM photograph obtained by afield-emission scanning electron microscope (FESEM). Specifically, amirror-polished sintered body sample was subjected to thermal etching,and the sample was observed with a field-emission scanning electronmicroscope (apparatus name: JSM-T220, available from JEOL Ltd.). Theaverage crystal grain size was calculated from the obtained SEMphotograph by the planimetric method.

(Yttria Concentration Distribution)

The maximum frequency was measured by using an FE-SEM/EDS (apparatusname: JSM-7600F, available from JEOL Ltd.) as follows. A surface of asintered body was observed with the SEM at a magnification of 24,000 toobtain a SEM observation image. A ratio of the yttrium concentrationrelative to the zirconium concentration (Y/Zr ratio) was obtained bymeasuring EDS spectra at 40,000 positions of the SEM observation image,and quantifying the intensities of characteristic X-rays of zirconiumand yttrium. The yttrium concentration distribution was obtained bydividing a range where the Y/Zr ratio exceeded 0.25% into ranges atintervals of 0.5%, and plotting the frequency in each of the yttriumconcentration ranges.

Prior to the measurement, the sintered body sample was subjected to Agsputter coating as a pretreatment.

(Crystal Phase)

XRD measurement of a sintered body sample was conducted by using acommon X-ray diffractometer (apparatus name: X'Pert PRO MPD, availablefrom Spectris Co., Ltd.). Conditions for the XRD measurement aredescribed below. The crystal phase of the sintered body sample wasidentified by subjecting the obtained XRD pattern to Rietveld analysisusing RIETAN-2000. The Rietveld analysis of the sintered body sample wasconducted under the assumption that the crystal phase included the Tphase, the T* phase and the C phase.

Radiation source: CuKα ray (X=0.1541862 nm)

Measurement mode: continuous scanning

Scan speed: 1°/min

Step width: 0.02°

Divergence slit: 0.5 deg

Scattering slit: 0.5 deg

Receiving slit: 0.3 mm

Measurement range: 2θ=10° to 140°

(Average Particle Size of Powder)

The average particle size of a zirconia powder was measured by using aMicrotrac particle size distribution analyzer (apparatus name: 9320-HRA,available from Honeywell Inc.).

A sample powder was suspended in distilled water to prepare a slurry,and the slurry was then subjected to a dispersion treatment with anultrasonic homogenizer (device name: US-150T, available from NIHONSEIKIKAISHA LTD.) for 3 minutes, as a pretreatment.

Herein, the term average particle size of a zirconia powder refers to adiameter of a sphere having the same volume as a particle having themedian diameter, which is a median value of a cumulative curve of aparticle size distribution represented on a volume basis, that is, aparticle size corresponding to 50% of the cumulative curve. The averageparticle size is a value measured by a particle size distributionmeasurement apparatus by a laser diffraction method.

(Crystallite Size of Powder Composition)

The crystallite size of a powder composition was obtained from a mainXRD peak by using the formula below.

Crystallite size=κλ/β cos θ

In the above formula, κ represents a Scherrer constant (κ=1), λrepresents the wavelength of the measured X-ray (λ=1.541862 Å when CuKαrays are used as a radiation source), β represents a full width)(°) athalf maximum of the main XRD peak, and θ represents the Bragg angle ofthe main XRD peak.

(Crystal Phase of Powder Composition)

The T+C phase ratio was calculated from an XRD pattern of a crystalphase of a powder composition by using the formula below.

T+C phase ratio (%)=100−fm (%)

In the above formula, fm represents a monoclinic phase ratio.

(Average granule Size of Granulated Powder)

The granule particle size of a granulated powder was obtained by a testmethod for sieve analysis.

Example 1

(Low Yttria Powder)

An aqueous solution of zirconium oxychloride was hydrolyzed to obtain ahydrated zirconia sol. Yttria was added to the hydrated zirconia solsuch that the yttria concentration became 2.5% by mole. Subsequently,the resulting mixture was dried and calcined to obtain anyttria-containing zirconia calcined powder. Regarding the calcinationconditions, the calcination was conducted in air at 1,160° C. for 2hours. The resulting calcined powder was washed with distilled water andthen dried. Thus, a 2.5 mol % yttria-containing zirconia powder wasobtained.

An α-alumina powder having an average particle size of 0.3 μm was mixedwith the 2.5 mol % yttria-containing zirconia powder such that thecontent became 0.05% by weight in terms of Al₂O₃ relative to the weightof the zirconia powder. Subsequently, distilled water was added theretoto prepare a slurry, and the slurry was ground and mixed. Thus, a slurrycontaining a low yttria powder of this Example was obtained. Thegrinding and mixing were conducted in a water solvent with a ball millby using zirconia balls having a diameter of 2 mm as grinding media, andthe mixing time was 24 hours.

The resulting low yttria powder had an yttria content of 2.5% by mole,an alumina content of 0.05% by weight, a BET specific surface area of11.2 m²/g and an average particle size of 0.42 μm.

(High Yttria Powder)

A 5.5 mol % yttria-containing zirconia powder that contained alumina inan amount of 0.05% by weight was obtained by the same method except thatyttria was added to a hydrated zirconia sol such that the yttriaconcentration became 5.5% by mole. This yttria-containing zirconiapowder was used as a high yttria powder of this Example.

The resulting high yttria powder had an yttria content of 5.5% by mole,an alumina content of 0.05% by weight, a BET specific surface area of10.1 m²/g and an average particle size of 0.40 μm.

(Powder Composition)

The slurry of the low yttria powder and the slurry of the high yttriapowder after the grinding and mixing were mixed in a ratio of 50% byweight:50% by weight, and the resulting mixture was sufficiently stirredto obtain a slurry containing a powder composition of this Example. Anorganic binder was added in an amount of 3% by weight to the slurrycontaining the powder composition, and the slurry was then spray-driedto obtain a granulated powder. The granulated powder had an averagegranule size of 44 μm and a light-duty bulk density of 1.28 g/cm³. Table1 shows evaluation results of the powder composition of this Example.

TABLE 1 Average T + C BET specific particle phase Crystallite Y₂O₃ Al₂O₃surface area size ratio size (mol %) (wt %) (m²/g) (μm) (%) (Å) Example1 4.0 0.05 10.7 0.41 68 380

(Sintered Body)

The resulting powder granules were molded by uniaxial pressing at 19.6MPa, and then molded by cold isostatic pressing (hereinafter, alsoreferred to as “CIP”) at 196 MPa to obtain a green body. The resultinggreen body was subjected to pressureless sintering in an air atmosphereat a sintering temperature of 1,450° C. and a temperature-increasingrate of 600° C./hr for a holding time of 2 hours. Thus, a zirconiasintered body of this Example having an yttria content of 4.0% by molewas obtained.

The crystal phase of the zirconia sintered body of this Example includedonly a tetragonal phase. The tetragonal phase included the T phase andthe T* phase, and the PVT ratio was 69.5%.

Table 2 shows evaluation results of the zirconia sintered body of thisExample.

Example 2

A zirconia sintered body of this Example having an yttria content of4.0% by mole was obtained by using the powder composition obtained inExample 1, and the same method as that used in Example 1 except that thesintering temperature was 1,500° C. Table 2 shows evaluation results ofthe zirconia sintered body of this Example.

Example 3

A zirconia sintered body of this Example having an yttria content of4.0% by mole was obtained by using the powder composition obtained inExample 1, and the same method as that used in Example 1 except that andthe sintering temperature was 1,550° C.

The crystal phase of the zirconia sintered body of this Example includedonly a tetragonal phase. The tetragonal phase included the T phase andthe T* phase, and the T*/T ratio was 81.8%. The maximum frequency of thezirconia sintered body of this Example was 6.9%. FIG. 1 shows theyttrium concentration distribution.

Table 2 shows evaluation results of the zirconia sintered body of thisExample.

Example 4

A green body was obtained by using the powder composition obtained inExample 1 and by the same method as that used in Example 1. The greenbody was pre-sintered at a pre-sintering temperature of 1,000° C. toobtain a pre-sintered body.

Sintering was conducted by the same method as that used in Example 1except that the resulting pre-sintered body was used instead of thegreen body and the sintering temperature was 1,600° C. Thus, a zirconiasintered body of this Example having an yttria content of 4.0% by molewas obtained.

The crystal phase of the zirconia sintered body of this Example includedonly a tetragonal phase. The tetragonal phase included the T phase andthe T* phase. The maximum frequency of the zirconia sintered body ofthis Example was 6.9%.

Table 2 shows evaluation results of the zirconia sintered body of thisExample.

TABLE 2 Total Three- Average Sintering light point crystal temp-Theoretical Relative transmit- bending grain erature density densitytance strength size (° C.) (g/cm³) (%) (%) (MPa) (μm) Example 1450 6.08099.85 44.6 876 0.49 1 Example 1500 6.080 99.87 44.5 935 0.66 2 Example1550 6.080 99.87 44.5 1020  0.90 3 Example 1600 6.080 99.87 44.8 1198 1.21 4

The above table shows that, in the manufacturing method according to thepresent invention, with an increase in the sintering temperature, theresulting sintered body exhibits high strength of 900 MPa or more, andfurther 1,000 MPa or more while having a translucency of 44% or more.

Example 5

(Low Yttria Powder)

A low yttria powder was obtained by the same method as that used inExample 1. The resulting low yttria powder had an yttria content of 2.5%by mole, an alumina content of 0.05% by weight, a BET specific surfacearea of 11.2 m²/g and an average particle size of 0.41 μm.

(High Yttria Powder)

A high yttria powder was obtained by the same method as that used inExample 1. The resulting high yttria powder had an yttria content of5.5% by mole, an alumina content of 0.05% by weight, a BET specificsurface area of 10.1 m²/g and an average particle size of 0.42 μm.

(Powder Composition)

The slurry of the low yttria powder and the slurry of the high yttriapowder after the grinding and mixing were mixed in a ratio of 55% byweight:45 by weight, and the resulting mixture was sufficiently stirredto obtain a slurry containing a powder composition of this Example. Anorganic binder was added in an amount of 3% by weight to the slurrycontaining the powder composition, and the slurry was then spray-driedto obtain a granulated powder. The granulated powder had an averagegranule size of 46 μm and a light-duty bulk density of 1.29 g/cm³. Table3 shows evaluation results of the powder composition of this Example.

TABLE 3 Average T + C BET specific particle phase Crystallite Y₂O₃ Al₂O₃surface area size ratio size (mol %) (wt %) (m²/g) (μm) (%) (Å) Example5 3.85 0.05 10.7 0.42 65 380

(Sintered Body)

A green body and a sintered body were obtained by the same method asthat used in Example 1. Table 4 shows evaluation results of the zirconiasintered body of this Example having an yttria content of 3.85% by mole.

Example 6

A zirconia sintered body of this Example having an yttria content of3.85% by mole was obtained by using the powder composition obtained inExample 5, and by the same method as that used in Example 4 except thatthe sintering temperature was 1,500° C. Table 4 shows evaluation resultsof the zirconia sintered body of this Example.

Example 7

A zirconia sintered body of this Example having an yttria content of3.85% by mole was obtained by using the powder composition obtained inExample 5, and by the same method as that used in Example 4 except thatthe sintering temperature was 1,550° C. Table 4 shows evaluation resultsof the zirconia sintered body of this Example.

TABLE 4 Three- point Sintering Theoretical Relative Total light bendingtemperature density density transmittance strength (° C.) (g/cm³) (%)(%) (MPa) Example 5 1450 6.083 99.84 44.9 901 Example 6 1500 6.083 99.8444.8 961 Example 7 1550 6.083 99.84 45.1 1078

Example 8

(Low Yttria Powder)

A low yttria powder was obtained by the same method as that used inExample 1 except that yttria was added to the hydrated zirconia sol suchthat the yttria concentration became 3.0% by mole, the calcinationtemperature was 1,100° C., and the mixing time in the ball mill was 16hours. The resulting low yttria powder had an yttria content of 3.0% bymole, an alumina content of 0.05% by weight, a BET specific surface areaof 13.1 m²/g and an average particle size of 0.40 μm.

(High Yttria Powder)

A high yttria powder was obtained by the same method as that used inExample 1. The resulting high yttria powder had an yttria content of5.5% by mole, an alumina content of 0.05% by weight, a BET specificsurface area of 10.1 m²/g and an average particle size of 0.41 μm.

(Powder Composition)

The slurry of the low yttria powder and the slurry of the high yttriapowder after the grinding and mixing were mixed in a ratio of 60% byweight:40 by weight, and the resulting mixture was sufficiently stirredto obtain a slurry containing a powder composition of this Example. Anorganic binder was added in an amount of 3% by weight to the slurrycontaining the powder composition, and the slurry was then spray-driedto obtain a granulated powder. The granulated powder had an averagegranule size of 43 μm and a light-duty bulk density of 1.27 g/cm³. Table5 shows evaluation results of the powder composition of this Example.

TABLE 5 Average T + C BET specific particle phase Crystallite Y₂O₃ Al₂O₃surface area size ratio size (mol %) (wt %) (m²/g) (μm) (%) (Å) Example8 4.0 0.05 11.9 0.41 77 370

(Sintered Body)

A green body and a sintered body having an yttria content of 4.0% bymole were obtained by the same method as that used in Example 1. Table 6shows evaluation results of the zirconia sintered body of this Example.

Example 9

A zirconia sintered body of this Example having an yttria content of4.0% by mole was obtained by using the powder composition obtained inExample 8, and by the same method as that used in Example 7 except thatthe sintering temperature was 1,500° C. Table 6 shows evaluation resultsof the zirconia sintered body of this Example.

Example 10

A zirconia sintered body of this Example having an yttria content of4.0% by mole was obtained by using the powder composition obtained inExample 8, and by the same method as that used in Example 8 except thatthe sintering temperature was 1,550° C. Table 6 shows evaluation resultsof the zirconia sintered body of this Example.

TABLE 6 Total Three- Average Sintering light point crystal temp-Theoretical Relative transmit- bending grain erature density densitytance strength size (° C.) (g/cm³) (%) (%) (MPa) (μm) Example 1450 6.08099.87 45.0 964 0.51 8 Example 1500 6.080 99.85 45.2 1106  0.74 9 Example1550 6.080 99.85 45.3 1126  0.90 10

Example 11

(Low Yttria Powder)

A low yttria powder was obtained by the same method as that used inExample 1. The resulting low yttria powder had an yttria content of 2.5%by mole, an alumina content of 0.05% by weight, a BET specific surfacearea of 11.2 m²/g and an average particle size of 0.39 μm.

(High Yttria Powder)

A high yttria powder was obtained by the same method as that used inExample 1. The resulting high yttria powder had an yttria content of5.5% by mole, an alumina content of 0.05% by weight, a BET specificsurface area of 10.1 m²/g and an average particle size of 0.40 μm.

(Powder Composition)

The slurry of the low yttria powder and the slurry of the high yttriapowder after the grinding and mixing were mixed in a ratio of 45% byweight:55 by weight, and the resulting mixture was sufficiently stirredto obtain a slurry containing a powder composition of this Example. Anorganic binder was added in an amount of 3% by weight to the slurrycontaining the powder composition, and the slurry was then spray-driedto obtain a granulated powder. The granulated powder had an averagegranule size of 42 μm and a light-duty bulk density of 1.27 g/cm³. Table7 shows evaluation results of the powder composition of this Example.

TABLE 7 Average T + C BET specific particle phase Crystallite Y₂O₃ Al₂O₃surface area size ratio size (mol %) (wt %) (m²/g) (μm) (%) (Å) Example4.15 0.05 10.6 0.40 85 380 11

(Sintered Body)

A green body and a sintered body having an yttria content of 4.15% bymole were obtained by the same method as that used in Example 1. Table 8shows evaluation results of the zirconia sintered body of this Example.

Example 12

A zirconia sintered body of this Example having an yttria content of4.15% by mole was obtained by using the powder composition obtained inExample 11, and by the same method as that used in Example 11 exceptthat the sintering temperature was 1,500° C. Table 8 shows evaluationresults of the zirconia sintered body of this Example.

Example 13

A zirconia sintered body of this Example having an yttria content of4.15% by mole was obtained by using the powder composition obtained inExample 11, and by the same method as that used in Example 11 exceptthat the sintering temperature was 1,550° C. Table 8 shows evaluationresults of the zirconia sintered body of this Example.

TABLE 8 Three- point Sintering Theoretical Relative Total light bendingtemperature density density transmittance strength (° C.) (g/cm³) (%)(%) (MPa) Example 11 1450 6.078 99.88 44.7 873 Example 12 1500 6.07899.90 45.1 929 Example 13 1550 6.078 99.88 45.5 1049

The above table shows that, with an increase in the sinteringtemperature, even the sintered bodies having an yttria content of morethan 4.0% by mole exhibit a strength of 870 MPa or more, further 900 MPaor more, and further 1,000 MPa or more.

Example 14

(Low Yttria Powder)

A low yttria powder was obtained by the same method as that used inExample 8. The resulting low yttria powder had an yttria content of 3.0%by mole, an alumina content of 0.05% by weight, a BET specific surfacearea of 13.0 m²/g and an average particle size of 0.40 μm.

(High Yttria Powder)

A high yttria powder was obtained by the same method as that used inExample 1. The resulting high yttria powder had an yttria content of5.5% by mole, an alumina content of 0.05% by weight, a BET specificsurface area of 10.0 m²/g and an average particle size of 0.41 μm.

(Powder Composition)

The slurry of the low yttria powder and the slurry of the high yttriapowder after the grinding and mixing were mixed in a ratio of 40% byweight:60% by weight, and the resulting mixture was sufficiently stirredto obtain a slurry containing a powder composition of this Example. Anorganic binder was added in an amount of 3% by weight to the slurrycontaining the powder composition, and the slurry was then spray-driedto obtain a granulated powder. The granulated powder had an averagegranule size of 45 μm and a light-duty bulk density of 1.29 g/cm³. Table9 shows evaluation results of the powder composition of this Example.

TABLE 9 BET specific Average Y₂O₃ Al₂O₃ surface area particle size (mol%) (wt %) (m²/g) (μm) Example 14 4.5 0.05 11.2 0.42

(Sintered Body)

A green body and a sintered body having an yttria content of 4.5% bymole were obtained by the same method as that used in Example 1. Table10 shows evaluation results of the zirconia sintered body of thisExample.

Example 15

A zirconia sintered body of this Example having an yttria content of4.5% by mole was obtained by using the powder composition obtained inExample 14, and by the same method as that used in Example 14 exceptthat the sintering temperature was 1,500° C. Table 10 shows evaluationresults of the zirconia sintered body of this Example.

Example 16

A zirconia sintered body of this Example having an yttria content of4.5% by mole was obtained by using the powder composition obtained inExample 14, and by the same method as that used in Example 14 exceptthat the sintering temperature was 1,550° C. Table 10 shows evaluationresults of the zirconia sintered body of this Example.

TABLE 10 Three- point Sintering Theoretical Relative Total light bendingtemperature density density transmittance strength (° C.) (g/cm³) (%)(%) (MPa) Example 14 1450 6.072 99.84 46.2 815 Example 15 1500 6.07299.85 46.2 850 Example 16 1550 6.072 99.82 46.4 830

The above table shows that the sintered bodies having an yttria contentof 4.5% by mole each have a strength of more than 800 MPa and exhibitstrength suitable for practical application to a dental material, thoughthe strength is lower than those of sintered bodies having an yttriacontent of 4.0% by mole.

Comparative Example 1

(Low Yttria Powder)

A low yttria powder was obtained by the same method as that used inExample 8. The resulting low yttria powder had an yttria content of 3.0%by mole, an alumina content of 0.05% by weight, a BET specific surfacearea of 12.9 m²/g and an average particle size of 0.42 μm.

(High Yttria Powder)

A high yttria powder was obtained by the same method as that used inExample 1. The resulting high yttria powder had an yttria content of5.5% by mole, an alumina content of 0.05% by weight, a BET specificsurface area of 9.9 m²/g and an average particle size of 0.43 μm.

(Powder Composition)

The slurry of the low yttria powder and the slurry of the high yttriapowder after the grinding and mixing were mixed in a ratio of 10% byweight:90% by weight, and the resulting mixture was sufficiently stirredto obtain a slurry containing a powder composition of this Example. Anorganic binder was added in an amount of 3% by weight to the slurrycontaining the powder composition, and the slurry was then spray-driedto obtain a granulated powder. The granulated powder had an averagegranule size of 43 μm and a light-duty bulk density of 1.28 g/cm³. Table11 shows evaluation results of the powder composition of thisComparative Example.

TABLE 11 BET specific Average Y₂O₃ Al₂O₃ surface area particle size (mol%) (wt %) (m²/g) (μm) Comparative 5.25 0.05 10.2 0.43 Example 1

(Sintered Body)

A green body and a sintered body having an Aria content of 5.25% by molewere obtained by the same method as that used in Example 1. The sinteredbody of this Comparative Example has a three-point bending strength of613 MPa and does not have a bending strength necessary for a dentalapplication, showing that a zirconia sintered body having an yttriacontent of more than 5.2% by mole has low mechanical strength.

Comparative Example 2

An aqueous solution of zirconium oxychloride was hydrolyzed to obtain ahydrated zirconia sol. A zirconia powder was obtained by the same methodas that used for the low yttria powder of Example 1 except that yttriawas added to the hydrated zirconia sol such that the yttriaconcentration became 4.0% by mole. This zirconia powder was used as azirconia powder of this Comparative Example.

The resulting low yttria powder had an yttria content of 4.0% by mole,an alumina content of 0.05% by weight, a BET specific surface area of11.2 m²/g and an average particle size of 0.41 μm.

A green body and a sintered body having an yttria content of 4.0% bymole were obtained by the same method as that used in ComparativeExample 1 except that the zirconia powder of this Example was used andthe sintering temperature was 1,550° C.

The crystal phase of the zirconia sintered body of this ComparativeExample included only a tetragonal phase. The tetragonal phase includedthe T phase and the T* phase, and the PVT ratio was 75.4%. The maximumfrequency of the zirconia sintered body of this Comparative Example was8.0%. FIG. 2 shows the yttrium concentration distribution.

The sintered body of this Comparative Example had a total lighttransmittance of 45.3% and a three-point bending strength of 870 MPa.This is lower than the strength of the sintered body of Example 11,which had an yttria content of 4.1% by mole and was obtained at the samesintering temperature. Furthermore, the bending strength of thisComparative Example had a minimum of 703 MPa and thus varied widely.These results show that the manufacturing method according to thepresent invention can provide a sintered body having a higher strengththan existing sintered bodies. In addition, the sintered body of thisComparative Example had a high T*/T ratio, showing that yttria wasdistributed more uniformly than that in the sintered bodies of Examples.

INDUSTRIAL APPLICABILITY

The method for manufacturing a zirconia sintered body according to thepresent invention can be provided as a method for manufacturing azirconia sintered body having both high translucency and high mechanicalstrength with high repeatability. Furthermore, the zirconia sinteredbody can be used as both a dental material for a front tooth and adental material for a back tooth.

What is claimed is:
 1. A method for manufacturing a zirconia sintered body, the method comprising molding a powder composition that has a yttria content of more than 3% by mole and 5.2% by mole or less and that contains a first zirconia powder having a yttria content of 2% by mole or more and 4% by mole or less and a second zirconia powder having a yttria content of more than 4% by mole and 6% by mole or less to obtain a green body; and sintering the green body to obtain a sintered body.
 2. The manufacturing method according to claim 1, wherein the powder composition contains alumina.
 3. The manufacturing method according to claim 1, wherein the powder composition has a BET specific surface area of 5 m²/g or more and less than 17 m²/g.
 4. The manufacturing method according to claim 1, wherein a weight ratio of the first zirconia powder to the second zirconia powder is 35% by weight:65% by weight to 65% by weight:35% by weight.
 5. The manufacturing method according to claim 1, wherein the first zirconia powder has an yttria content of 2% by mole or more and 3.5% by mole or less, and the second zirconia powder has an yttria content of 4.5% by mole or more and 5.7% by mole or less.
 6. The manufacturing method according to claim 4, wherein a sintering temperature in the sintering step is 1,400° C. or higher and 1,600° C. or lower.
 7. A powder composition comprising a yttria content of more than 3% by mole and 5.2% by mole or less and that contains a first zirconia powder having a yttria content of 2% by mole or more and 4% by mole or less and a second zirconia powder having an yttria content of more than 4% by mole and 6% by mole or less.
 8. The powder composition according to claim 7, wherein a content ratio of the second zirconia powder to the first zirconia powder is second zirconia powder:first zirconia powder=20% by weight:80% by weight to 80% by weight:20% by weight.
 9. The powder composition according to claim 7, wherein an alumina content is 0% by weight or more and 0.1% by weight or less.
 10. The powder composition according to claim 7, wherein a crystallite size of the powder composition is 200 Å or more and 400 Å or less.
 11. The powder composition according to claim 7, wherein a BET specific surface area is 5 m²/g or more and less than 17 m²/g.
 12. A method for producing a green body comprising forming a powder composition that has an yttria content of more than 3% by mole and 5.2% by mole or less and that contains a first zirconia powder having an yttria content of 2% by mole or more and 4% by mole or less and a second zirconia powder having an yttria content of more than 4% by mole and 6% by mole or less, to obtain a green body.
 13. A method for producing a pre-sintered body comprising heat-treating a powder composition that has an yttria content of more than 3% by mole and 5.2% by mole or less and that contains a first zirconia powder having an yttria content of 2% by mole or more and 4% by mole or less and a second zirconia powder having an yttria content of more than 4% by mole and 6% by mole or less, to obtain a pre-sintered body. 